I’ve been to Gallipoli during March when there was nobody there. It was fantastic because the group I was with were the only people about. It was really great to be able to wander around the site in virtual silence.
If you are going to go, I’d highly recommend going with TJ’s Tours. They’re run out of Eceabat which is just a short drive away. TJ is a Turkish guy who married an Australian woman and he’s really knowledgeable about the area and the history from both sides of the fence. I can’t give the guy enough praise.